A liquid crystal display (LCD) modulates light flow by rotating the alignment of liquid crystal molecules to control the amount of light which enters a polarizing filter film with a vertical (or horizontal) axis and passes through another polarizing filter film with a horizontal (or vertical) axis. The liquid crystal molecules are aligned between the two polarizing filter films and the axis of the filters may be perpendicular or parallel from each other. Here, the rotation of the liquid crystal molecules is modulated by the electrical setting because each liquid crystal molecule is aligned along with an electric field which can be made by the electrical setting for an individual pixel. Various kinds of the electrical settings have been developed, but generally, the rotation angle and speed are decided by the voltage level of the electric field. Thus, the voltage decides the gray scale level of each LCD pixel.
Generally, the voltage for the gray scale level is called a driving or data driving voltage. FIG. 1 shows the relationship between the driving voltage and the gray scale level. As shown in FIG. 1, the driving voltage may have either positive or negative polarity to display a same gray scale because the liquid crystal may rotate to either direction with the same manner of the light control. Usually, the voltage which is higher than the common voltage (V0) becomes a voltage of positive polarity, and a voltage which is lower than V0 becomes a voltage of negative polarity.
One of the key issues with LCDs is that the rotation speed of liquid crystal molecules is relatively slow, below the image refresh rate (frame rate). For example, in the case of Amorphous Silicon (a-Si) TFT-LCD, the mobility of a-Si is approximately 0.3-0.5 (cm/Vs), which is not sufficient when a scene is changing fast or there is a fast moving objects on the scene (the scene is blurred or the object can be disappeared from the scene). Usually, each LCD pixel is modeled as a capacitor where the full rotation time of liquid crystal molecules is considered as a full charging time of the capacitor model. Thus, the above issue is generally known as a “short charge time” or “short response time” of a pixel. Also, sometimes, the voltage which is charged in the capacitor model is called as a potential.
Various solutions have been developed to solve the short charge time problem. One of the solutions is compensating the charge time of the pixel by overdriving the pixel with initial high pre-charge voltage. Here, the initial high voltage should be higher than the real data voltage of the target gray scale level. After the initial high pre-charge voltage, the voltage should be modulated as the gray scale of the pixel approaches the target level. The initial high voltage enables the rotation of liquid crystals to be faster, and then the voltage should be eased off as it reaches the target gray scale level.
FIG. 2 shows a comparison of the cases where there is a short charged pixel without the initial high pre-charge voltage and a fully charged pixel with the initial high pre-charge voltage. The left case of FIG. 2 shows that the pixel is not charged enough to display the target gray scale level due to the limited horizontal period (1H: one horizontal period) and the characters are blurred on the screen. On the other hand, the one on the right shows that the pixel is charged enough to display the target scale level within the same horizontal period (1H) and the characters on the screen are sharper than the left one through applying the initial high pre-charge voltage.
However, this conventional initial high pre-charge voltage has some disadvantages. First, the conventional initial high pre-charge voltage requires relatively high voltage. Further, in the conventional initial high pre-charge voltage, too much high voltage may cause the pixel to display a wrong target gray scale level and the voltage needs to be reduced before this happens. Also, a data driver with double speed is required because the horizontal period (1H) should be divided into a pre-charge period and a real data period for a pixel.
Also, as shown in FIG. 3, when one color image which is an intermediate gray scale level is displayed after both white and black (maximum and minimum gray scale levels) are displayed at the same frame during some period, an “after image” occurs on a boundary between the white and black images. The detail explanation why the after image occurs is to be discussed below.
Therefore, exemplary objects of the present disclosure involve solving the above problems by compensating the pixel charge time with half driving speed of the conventional driving method without the initial high charging voltage. Also, an additional object of the present disclosure is to solve the after image problem.